The present invention relates to a process for the preparation of isocyanates by reacting primary amines with phosgene in a reactor, the reaction discharge being present in the form of a suspension which contains the isocyanate to be prepared, as a liquid, and carbamyl chlorides as a solid, and working up the suspension in a film evaporator, and a production plant for carrying out this process.
Numerous processes for the preparation of isocyanates by reacting primary amines with phosgene are known. For example, U.S. Pat. No. 3,140,305 describes a process for the preparation of substantially pure aromatic diisocyanate by reaction of amines with phosgene and subsequent distillation of the reaction products.
It is furthermore known that the use of a large excess of phosgene relative to the amino groups used leads to high selectivities with respect to the isocyanate prepared and may thus have a decisive effect on the cost-efficiency of the process. However, the phosgene hold-up of the plant increases with increasing ratio of phosgene to amino groups, a very short phosgene hold-up of the plant being desirable owing to the toxicity of phosgene.